Over the years, a number of different systems involving power line communication (PLC) have been suggested. The greatest benefit of these systems is that they use existing power lines as a communication medium instead of using costly wireless equipment or dedicated communication wiring and hardware. For instance, installation of a dedicated wiring in an existing building can be costly, difficult to achieve and not desirable, especially if it involves the destruction of walls, ceilings or the like. This provides a strong motivation to use existing power lines for communication purposes in addition to their use for power distribution, thereby taking advantage of the already existing electrical network that is present in almost any building, whether industrial, commercial or residential.
While the idea of using PLC has resulted in a number of different constructions, none has been found completely satisfactory and acceptance was thus limited. Most limitations of prior systems result from inherent obstacles encountered when communicating over power lines. Since power lines were not intended to provide a communication medium, problems and impairments must be overcome in order to develop a reliable communication system. Very often, these problems have led to inconsistent and unreliable performance in previous PLC systems. The major challenges in the development of this technology include various noise sources, such as white noise (noise generated by electrical appliances), harmonic distortion, and signal attenuation (attenuation due to lengthy connection and varying impedance). These problems are not significant when distributing electricity at 60 hertz. However, when transmitting communication signals in the kilohertz range and above, they can become obstacles.
The object of the present invention is to provide satisfactory PLC between two communication control devices, this being achieved in a way which reduces, if not obviate, the difficulties and disadvantages of the existing systems through a number of improvements.
In the present invention, the power line communication system comprises a transmitter and a receiver interfaced between each communication control device and the power line. At the receiver, communication signals are amplified and transmitted to the power line. At the other end, the receiver receives the communication signals from the power line, preferably conditions the signals and then amplifies them before they are sent to the corresponding communication control device.
A first aspect of the present invention is that the output impedance of the transmitter can be selectively switched between an enabled and disabled state. Also, to compensate for different medium conditions such as loading of the power line by other appliances or surrounding noise, the output impedance of the transmitter is advantageously capable of automatically switching between two steps of low impedance. The first step allows for reliable transmission to account for other impedances on the power line, while the other decreases noise seen on the power line.
Another aspect of the present invention is that the receiver advantageously uses a logarithmic amplifier to preserve the phase of the signal and therefore keeps the information stored in the x-axis intercepts.
A further aspect of the present invention is to provide the receiver with a carrier sense function. With this carrier sense function, when in the receiving mode, a reference signal mixes with the incoming communication signal. If the received signal on the power line is lower than the reference signal, correct demodulation of the reference signal is performed by the communication control device and the received signal is seen as noise. However, if the received signal on the power line is higher than the reference signal, demodulation errors occur at the communication control device, thereby indicating that the received signal is an actual message.
Another aspect of the present invention is to provide the option of communicating signals through the ground and neutral wires, which could be useful in certain circumstances. For instance, this scheme could be used if the risk of signal degradation through the phase and neutral lines becomes too high.